The known composite forming fabrics comprise two essentially separate woven structures, each of which includes its own sets of warps and wefts, and each of which is woven to a pattern selected to optimise the properties of each of the layers. The paper side layer should provide, amongst other things, a minimum of fabric wire mark to, and adequate drainage of liquid from, the incipient paper web. The machine side layer should be tough and durable, provide a measure of dimensional stability to the forming fabric so as to minimize fabric stretching and narrowing, and be sufficiently stiff to minimize curling at the fabric edges. Numerous fabrics of this type have been described, and are in industrial use.
The two layers of the known composite forming fabrics are interconnected by means of either additional binder yarns, or intrinsic binder yarns. Additional binder yarns serve mainly to bind the two layers together; intrinsic binder yarns both contribute to the structure of the paper side layer and also serve to bind together the paper and machine side layers of the composite forming fabric. The paths of the binder yarns are arranged so that the selected yarns pass through both layers of the fabric, thereby interconnecting them into a single composite fabric.
In these known composite fabrics, additional weft binder yarns were generally preferred over intrinsic weft binder yarns, as they were believed to cause fewer discontinuities in the paper side surface of the composite fabric. Recently, both single and paired intrinsic warp or weft binder yarn arrangements have been proposed. However, intrinsic weft binder yarns have been found to cause variations in the cross-machine direction mesh uniformity. Composite fabrics in which intrinsic weft binder yarns are incorporated have been found to be susceptible to lateral contraction under the tensile load placed upon them in a papermaking machine. These intrinsic weft binder yarns have also been found to be susceptible to internal and external abrasion, leading to catastrophic delamination of the composite fabric. Further, due to the necessity of having to weave into the fabric structure additional weft yarns to form the paper side layer, and to bind the paper side layer and machine side layer together, these fabrics are expensive to produce.
More recently it has been proposed to use intrinsic warp binder yarns in pairs or triplets, so as to overcome at least some of these disadvantages. Fabrics of these two types are described by Vöhringer in U.S. Pat. No. 5,152,326 (pairs); by Stone et al. in U.S. Pat. No. 6,240,973 (triplets); and by Johnson et al. in U.S. Pat. No. 6,202,705 (triplets).
The use of pairs offers the advantages that the two warp binder yarns can be incorporated in sequence in successive segments of an unbroken warp path in the paper side surface, and that there is more flexibility of choice for the locations at which each member of the pair interlaces with the machine side layer wefts. It is thus possible to optimise the paper side surface to some extent, for example to reduce wire marking of the incipient paper web, and to improve the machine side layer wear resistance of the fabric, essentially by increasing the amount of material available to be abraded away before catastrophic failure, usually by delamination, occurs. In these fabrics using pairs of warp binder yarns, the paper side layer and machine side layer each have separate weft yarn systems, one of which completes the paper side layer weave, and the other of which completes the machine side layer weave.
In the following discussion of this invention, it is to be understood that in a notation such as “2×2” the first number indicates the number of sheds required to weave the pattern, and the second number indicates the number of wefts in the pattern repeat. Thus a 2×2 pattern requires two sheds, and there are two wefts in the pattern repeat.
As disclosed by Stone et al. and by Johnson et al. the use of warp triplets offers the advantage that the fabric structure can be simplified, in that the fabric can be woven with only three sets of yarns: a paper side layer set of wefts, a machine side layer set of wefts and a single set of warps which contributes to the structure of both layers. It is possible to weave a fabric having acceptable paper making properties by utilizing triplets of warp yarns so that each member of the triplets interweaves separately in sequence with the paper side layer wefts, and so that the members of the triplets interlace in pairs with the machine side layer wefts. The pairs of warp yarns when interlaced with the machine side layer weft yarns cause these yarns to bow outwards somewhat, towards the machine side surface of the fabric. This provides a wear plane which increases fabric wear potential, which increases fabric life.
The use of triplets woven in pairs with the machine side layer wefts provides a forming fabric having reduced susceptibility to cross-machine direction variations in the paper side layer mesh uniformity, less susceptibility to dimpling of the paper side surface, and better resistance to lateral contraction than comparable fabrics of the prior art. It is possible to weave some of these warp triplet fabrics from a single warp beam, because all of the warp yarns follow essentially similar paths, which have equal path lengths within the weave structure.
However it has been found that composite forming fabrics woven using triplet sets of warp yarns are still susceptible to dimpling of the paper making surface of the paper side layer. It appears that, as the warp yarns pass from one surface of the fabric to the other, eg from the surface of paper side layer to the surface of the machine side layer, they may introduce some non-uniformity into the otherwise regular spacing of the paper side layer weft yarns. This creates variations in both the shape and frame lengths of the drainage openings in the paper side layer of the forming fabric, which results in variation of the drainage properties of the fabric. These variations can introduce an unacceptable level of marking (so-called “wire mark”) into the paper product being made.
It has now been found that this level of variation can be at least mitigated by interlacing each member of a warp triplet set on its own with a machine side layer weft yarn. When this step is taken, it is then possible for each member of each of the triplet sets to follow the same path within the weave pattern, thus providing more uniform location of the interlacing points. This has the result that the paper side layer surface characteristics are improved which in its turn provides more uniform formation in the paper product.
Additionally, we have found that by careful choice of the warp yarn material used in the fabrics of this invention, it is possible to weave a fabric having a high paper side surface open area with sufficient drainage area to rapidly drain the embryonic sheet into the central plane of the fabric structure, without sacrificing critical mechanical properties of the fabric, in particular its elastic modulus. In the central plane and machine side layer of the fabric, where yarn density is higher, fluid drainage appears to be retarded slightly, thus providing opportunity for pressure pulses caused by the supporting foils and blades of the forming section to maximise formation benefits.
We have found that relatively smaller diameter, high elastic modulus yarns can be used in place of conventional, relatively larger diameter polyethylene terephthalate(PET) yarns as the warp yarns in the fabrics of this invention, to provide equivalent mechanical strength properties. It is thus possible to use these smaller diameter yarns to provide the fabric with a relatively high paper side layer drainage area at a lower warp yarn density in the paper side surface. This in turn allows for the use of a higher number of cross machine direction weft yarns than would otherwise be possible in the paper side surface so as to increase fiber support in the sheet, thereby improving formation. These extra weft yarns will in turn contribute to overall fabric stiffness and stability which are necessary for dependable service life (i.e. “runnability”)
The fabrics of this invention are thus able both to drain fluid from the sheet more rapidly than would be possible in comparable fabrics woven using larger warp yarns, and to provide increased support for the papermaking fibers in the stock so as to improve overall formation. Use of these high elastic modulus yarns also improves the resistance of the fabrics to damage from high pressure showers such as are used to clean them during use. Further, these smaller diameter, high elastic modulus warp yarns will be recessed to an extent into the machine side surface of the fabric due to both machine side layer weave design and the heatsetting conditions used to process the fabric following weaving. Following heatsetting, the weft yarns on the machine side of the fabric tend to bow, or crimp outwardly, forming a wear plane which serves to protect the warp yarns from abrasion during use. This feature serves to increase further the service life of these fabrics.